1. Field of the Invention
The present invention relates to an optical disk device, and in particular to optimization of a writing power when data is written onto a recordable optical disk.
2. Description of the Related Art
When data is to be written onto a recordable optical disk such as a CD-R, CD-RW, DVD-R, or DVD-RW disk, test data is repeatedly written as a test in a predetermined region (PCA area) of the optical disk while the writing power is varied, the test data is replayed, a writing power in which the quality of the replayed signal is maximized is selected as the optimum writing power, and the data is written (a process commonly referred to as optimum power control (OPC)).
For example, Japanese Patent Laid-Open Publication No. 2000-137918 discloses such a method in which test data is written as a test in the PCA area of an optical disk while varying the writing power, jitter value of the written test data is measured, and the writing power in which the jitter value is minimized is determined as the optimum writing power.
In a CD-R or CD-RW system, signals having lengths from 3T to 11T are written and in a DVD-R or DVD-RW system, signals having lengths from 3T to 14T are written. Therefore, when the writing power is to be optimized through OPC as described above, it is desirable to optimize the writing power using test data of length 3T to 11T or 3T to 14T.
However, in general, it is difficult to write the shortest signal, the signal with length 3T (“3T signal”), and therefore, the signal may sometimes be missing because of a writing error. Because the jitter value is calculated from the edge difference between the rising edge and the falling edge of the signal and a reference clock, even when the 3T signal is missing, for example, the jitter value is measured at the rising edge or the falling edge of the remaining signals of 4T to 11T or 4T to 14T. As a result, the jitter value appears to be small although the test data is not normally written, and there had been a problem in that the optimum writing power is selected based on this apparently small jitter value.
This problem will now be described in more detail. FIGS. 12A through 12E show a test data string, rising and falling edges of a replayed binarized signal based on which jitter value is calculated, the reference clock, and difference between the rising and falling edges and the reference clock. FIG. 12A shows the actual data string of the test data. As shown in FIG. 12A, it is assumed that the actual test data has a data string of 6T (mark), 3T (space), 3T (mark), 3T (space), 4T (mark), 3T (space), 3T (mark), 9T (space) periods. Here, “mark” indicates a marking period in which laser light of a writing power is illuminated and a pit is actually formed on the optical disk and “space” indicates a space period in which laser light of a bias (replay) power is illuminated between the marking periods of writing power and no pit is formed. In a DVD-R system, however, a multi-pulse is employed for forming the pit and the bias level between the multi-pulses is “mark” and not “space”.
FIG. 12B shows a replayed signal obtained by replaying the written test data. FIG. 12B also shows a threshold value for binarizing the replayed signal. Because the 6T (mark) period and 4T (mark) period have relatively large pulse width, sufficient energy can be illuminated to write a pit, and, thus, the level of the replayed signal is greater than the threshold value. However, the 3T (mark) period has a small pulse width and, when the writing power is insufficient, the level of replayed signal may sometimes fall short of the threshold value because a pit is not adequately formed. In such a case, when the replayed signal is binarized using the threshold value, the 3T (mark) period is not captured and will be missing.
FIG. 12C shows a binarized signal obtained by binarizing the replayed signal. Because3T (mark) period is missing, a data signal of 6T (mark), 9T (space), 4T (mark), and 15T (space) periods, which differs from the original test data, is retrieved. When such a binarized signal and a reference clock as shown in FIG. 12D are compared and the edge difference between the rising and falling edges of the binarized signal and the reference clock is detected, a signal as shown in FIG. 12E would be obtained. As shown, because the 3T (mark) period is missing, the edge difference in a corresponding portion present in the original data will not be detected. As a result, the overall edge difference, that is, the jitter value, appears smaller than it actually is. Therefore, jitter value for the writing power appears to be smaller than the actual value, and thus, there is a problem in that although in reality, the writing power is insufficient, this power may be erroneously determined as the optimum writing power.
On the other hand, there may be cases wherein, instead of missing a 3T (mark) signal, the 3T (mark) signal is excessively recorded due to excessive writing power, and is detected as data of a length 4T or more. In such a case also, although in reality, the test data is not normally written, the jitter value appears smaller.
FIGS. 13A–13E show a specific example of such a case. As shown in FIG. 13A, the actual test data contains 6T (mark), 3T (space), 3T (mark), 3T (space), 4T (mark), 3T (space), 3T (mark), and 9T (space) periods.
FIG. 13B shows a replayed signal. When the 3T mark signal is excessively recorded, the portion in which the level of the replayed signal exceeds the threshold value level becomes larger, resulting in a binarized signal of 6T (mark), 3T (space), 4T (mark), 2T (space), 4T (mark), 3T (space), 4T (mark), and 8T (space) periods as shown in FIG. 13C. As can be seen, the actual 3T (mark) period is detected as a 4T (mark) period and the actual 3T (space) period appears to be a 2T (space) period. In this case, when the edge difference is detected between the binarized signal and the reference clock, the edge difference appears smaller than that of a 3T (mark) period, resulting in smaller apparent jitter value. Because in OPC, the writing power with minimum jitter value is determined as the optimum writing power, a writing power may be erroneously determined as the optimum writing power even when the writing power is actually excessive.
FIG. 14 shows a relationship between the pulse width of 3T (writing power) and jitter value. In FIG. 14, the horizontal axis represents the pulse width (writing power) of 3T (mark) period and the vertical axis represents the jitter value. Normally, when a 3T (mark) period is written, the jitter becomes a minimum at a certain point and increases as the pulse width moves away from the minimum point. However, when the pulse width of a 3T (mark) period is small (insufficient writing power), as in the region I in FIG. 14 and is missing, the jitter value becomes smaller because the edge difference of the actual 3T (mark) period does not appear as described above. On the other hand, when the pulse width of 3T (mark) period is excessively increased (excessive writing power) as in region II in FIG. 14, a 3T (mark) period appears as a 4T (mark) period as described above and jitter value again appears smaller as the 3T (mark) period is not present. Therefore, as shown in FIG. 14, the characteristic appears to have a shape with two peaks. In this case, the writing power where the jitter value is minimized cannot be calculated, and a writing power which is smaller than the actual optimum writing power or a writing power which is greater than the actual optimum writing power may be erroneously determined as the optimum writing power, resulting in a problem in that the actual data cannot be written with high quality.